1. Field of the Invention
The present invention relates generally to an apparatus, method and computer program for detecting an echo signal in the frequency domain that may be present in a Digital Video Broadcasting-Terrestrial (DVB-T) receiver.
2. Description of the Related Art
In digital television (DTV), two data transmission methods may be used: a Vestigial Side Band (VSB) method, which is a single carrier conversion method, and a Coded Orthogonal Frequency Division Multiplexing (COFDM) method, which is a multi-carrier conversion method. A DVB-T system adopting the COFDM data transmission method may be recognized as a next-generation terrestrial DTV transmission system and has been used for experimental broadcasting in Europe. The DVB-T system may be used in the global terrestrial digital market along with the U.S. terrestrial broadcasting standard. A DVB-T modulation/demodulation method may use an OFDM method for a terrestrial wave. In general, single carrier modulation/demodulation methods may transmit information in series, where the OFDM method may transmit information dispersively at a plurality of different frequencies. Therefore, the OFDM method may be suitable for multi-path channel communications.
The DVB-T receiver may provide estimation of a channel for a modulated signal, which may be required for coherent demodulation of the modulated signal received. However, coherent demodulation may not be achieved until a channel estimation unit of the DVB-T receiver estimates both a channel and any residual phase errors that may be present.
FIG. 1 is a block diagram of a conventional DVB-T receiver. Referring to FIG. 1, the conventional DVB-T receiver includes an Analog-to-Digital (A/D) converter 101, an I/Q generator 102, a fast Fourier transform generator (FFT) 103, an equalizer 104, a Forward Error Correction (FEC) unit 105, a sampling frequency synchronization unit 106, and a carrier synchronization unit 107.
An Intermediate Frequency (IF) input signal of the conventional DVB-T receiver may be obtained from receiving an OFDM signal via an antenna and converting the OFDM signal into the IF signal using a tuner (not shown). The IF signal may be converted into a digital signal in the A/D converter 101 and may be output to the I/Q generator 102. The digital signal input to the I/Q generator 102 may contain only an In-phase (I) component. The I/Q generator 102 may convert the input digital signal into a complex signal containing an (I) component and a Quadrature (Q) component. The FFT 103 may perform a Fast Fourier Transform ‘FFT’ on the signal output from the I/Q generator 102. The equalizer 104 may provide compensation for a carrier signal which may be distorted due to channel distortion, using the signal output from the FFT 103. The FEC unit 105 may detect errors in the data of the OFDM signal using a designated error detection method, and may perform Forward Error Correction ‘FEC’ on the detected errors.
The sampling frequency synchronization unit 106 may transmit a signal to the I/Q generator 102, which may enable sampling frequency synchronization using the signal output of the FFT 103. The carrier synchronization unit 107 may transmit a signal to the I/Q generator 102, which enables carrier synchronization using the signals output from the I/Q generator 102 and the FFT 103.
The equalizer 104 may estimate characteristics of a channel for transmission of an OFDM signal by using scattered pilot signals, and may compensate for the estimated characteristics of the channel.
Application of the scattered pilot signals may be further defined in the DVB-T standard, which requires channel estimation through interpolation. For example, a plurality of Channel Impulse Response (CIR) samples may be obtained using already known scattered pilot signals, and interpolation may be performed on these samples in the time and frequency domains for channel estimation. Two-Dimensional (2D) interpolation, with respect to time and frequency, may include performing interpolation in the time domain based on characteristics of a channel transfer function CIRT(ƒ), and interpolation performed in the frequency domain based on characteristics of a channel transfer function CIRF(ƒ).
Interpolation may be optimized using Equation (1) wherein a Minimum Mean Square Error (MMSE) may be applied to a channel estimation value Ĥm(k) and a real CIR value Hm(k) of an mth subcarrier of a kth OFDM symbol respectively:∂CIR2=E{(Ĥm(k)−Hm(k))2}  (1)to calculate ∂CIR2, a 2D representation of interpolation. In calculating the 2D interpolated CIR, the correlation functions of the CIR in the time/frequency domain, noise deviation that disturbs the CIR samples, the total number of the scattered pilot signals, and the patterns of the scattered pilot signals may be considered.
Channel estimation using the MMSE may be achieved with a 2D Wiener filter, however computation may be complex when using the 2D Wiener filter. The use of a channel transfer function in the frequency domain CIRF(ƒ) whose range falls within 0≦τn≦TGl and a channel transfer function in the time domain CIRT(ƒ) that satisfies |ƒDn|≦|ƒDmax| may simplify the process of channel estimation. For example, τn denotes a multi-path delay in an nth path, TGl denotes guard intervals of an OFDM signal, |ƒDn| denotes a Doppler spread along the nth path, and |ƒDmax| denotes a maximum Doppler spread that is limited to a time interval of an OFDM symbol.
Alternatively, the 2D Wiener filter may be replaced with two one-dimensional (1D) filters that continuously operate and perform time-domain interpolation and frequency-domain interpolation. The two 1D filter approach may reduce the complexity of the calculations. The two 1D filters may be implemented with varying CIR functions and associated intervals.
Frequency-domain interpolation may use an equivalent time-domain Low-Pass Filter (LPF) with a bandwidth denoted TGl, similar to a Wiener filter or a filter based on Inverse Discrete Fourier Transform/Discrete Fourier Transform (IDFT/DFT) operations. The LPF may provide the frequency-domain interpolation process with a reliable channel estimation result.
However, when the guard interval TGl is too small, that is the channel transfer function CIRF(ƒ) and the bandwidth of the equivalent time-domain LPF for the frequency-domain interpolation are matched too closely, a system may become vulnerable to a multi-path channel containing an echo component whose delay is larger than the guard interval TGl 